1. Field of the Invention
The present invention generally relates to image forming apparatuses and to belt drivers that perform speed correction control of belts, such as carrying belts and transfer belts.
2. Description of the Related Art
In these days, based on the requirements of the marketplace, there are many electrophotographic apparatuses that output color images, such as color copying apparatuses and color printers. Especially nowadays, it is expected for electrophotographic apparatuses forming full-color images to achieve the speed as fast as that of electrophotographic apparatuses forming black-and-white images. Thus, mainstream printers are printers that are provided with a developing apparatus for each of a plurality of photo conductors, form a monochrome toner image on each of the photo conductors, sequentially superimpose the monochrome toner images, and transfer the images to paper so as to record a color image thereon.
Tandem printers include direct transfer printers and indirect transfer printers. As shown in FIG. 1, in a direct transfer printer, images of respective colors formed on photo conductors 2K, 2M, 2C and 2Y by image forming means 1K, 1M, 1C and 1Y are sequentially superimposed on paper that is carried by a transfer belt 4 by transfer apparatuses 3K, 3M, 3C and 3Y, respectively. As shown in FIG. 2, in an indirect transfer printer, images of respective colors formed on photo conductors 5K, 5M, 5C and 5Y are sequentially superimposed on a transfer belt 7 by transfer apparatuses 6K, 6M, 6C and 6Y, respectively. Then, the images on the transfer belt 7 are simultaneously transferred to paper by a secondary transfer apparatus 8.
In each of the direct transfer printer and the indirect transfer printer, the images of the respective colors are transferred to the transfer belt 4(7) at different positions thereon. Thus, image data of the respective colors are output to the image forming means 1K, 1M, 1C and 1Y with respective delays corresponding to the intervals among the photo conductors. For this reason, when the moving speed of the transfer belt 4(7) is not constant, a shift occurs to the transfer positions to which the images of the respective colors are to be transferred. As a result, color shift occurs with the formed image. Therefore, it is necessary for tandem electrophotographic apparatuses to control with a high accuracy the speed of the transfer belt such that it is constant at all times.
However, it is structurally difficult for the transfer belt 4(7), which is used here, to realize a uniform thickness. Especially, the thickness variation tends to be generated in the longitudinal direction (direction of movement). In a case where the thickness of the transfer belt 4(7) is not uniform, even if the rotating speed of a drive shaft driving the transfer belt 4(7) is controlled to be constant, periodic speed variation occurs in the surface speed of the transfer belt 4(7). For this reason, tandem printers have a problem in that shifts occur in the transfer positions to which images of respective colors are to be transferred, which tends to result in out of color registration of a formed image.
In order to correct the speed variation that is synchronized with the rotation period of the transfer belt, a method is conceived which measures in advance the thickness variation or the speed variation for one rotation of the transfer belt, and corrects the rotating speed of the drive shaft of the transfer belt or the write timings of respective colors based on the information, so as to reduce the speed variation of the transfer belt. However, it is impossible to correct the speed of the transfer belt unless it is determined which position of the transfer belt is being driven. Hence, a method is conceived that marks a home position mark 11 representing a reference point on the transfer belt 4(7) as shown in FIG. 3, and detects the home position mark 11 by a sensor 12 as shown in FIG. 4, so as to detect a home position (hereinafter referred to as a “HP”) of the transfer belt 4(7).
When forming an image, it is possible to control the speed of the transfer belt 4(7) by starting the running of the transfer belt 4(7), thereafter starting a detecting process of the home position mark 11 by the sensor 12, and, when the home position mark 11 on the transfer belt 4(7) is detected by the sensor 12, starting a speed correction control of the transfer belt 4 (7) based on speed correction data that are prepared in advance (refer to Japanese Laid-Open Patent Application No. 8-328443, for example).
Japanese Laid-Open Patent Application No. 2001-51479 describes an image forming apparatus that includes: a plurality of image forming means; an endless movable body that places and conveys a recording medium such that respective images formed by the plurality of image forming means are transferred to the recording medium at the transfer positions; a position detecting means for detecting a placing position at which the recording medium is placed on the endless movable body; and correcting means for correcting the image formation timings of the plurality of image forming means based on a detection output of the position detecting means.
Japanese Laid-Open Patent Application No. 11-249380 describes a color image forming apparatus that forms a color image on a recording medium by sequentially superimposing and transferring, to the recording medium carried by the carrying belt, images formed by a plurality of electronic process parts, including image carriers, arranged along a carrying belt. The color image forming apparatus includes: a light emitting element illuminating the carrying belt; a slit through which the light transmitted through or reflected by the carrying belt passes; a light receiving element receiving the light that passes through the slit; a shift amount detector for detecting a position shift amount of a detection pattern, the shift amount detector having means for processing a signal from the light receiving element; pattern forming means for activating each of the electronic process parts such that the same electronic process part forms a plurality of pairs of toner marks as the position shift amount detection pattern for each color with the shape corresponding to the slit, each pair including two toner marks of the same color and the same shapes, one toner mark of the pair is distant from the other toner mark for the half peripheral length of the image carrier, and adjacent toner marks among the plurality of pairs of toner marks are formed with a predetermined distance (hereinafter referred to as a “mark pitch”) therebetween. The toner marks formed by the pattern forming means are detected by the shift amount detector. The shift amounts among respective images formed by the electronic process parts are detected. Among the plurality of pairs of toner marks, when focusing attention to the first formed toner mark (hereinafter referred to as a “reference mark”) of two toner marks constituting a pair of toner marks formed by a predetermined reference color, a toner mark for correction (hereinafter referred to as a “correction mark”) is formed while setting the target to the position that is distant for one peripheral length of the image carrier that forms the reference mark. The correction mark is also the target to be detected by the shift amount detector.
In tandem printers, the peripheral length of the transfer belt 4 (7) becomes structurally long. Thus, depending on the stop position of the transfer belt 4(7), there is a problem in that it takes a long time for the sensor 12 to detect the home position mark 11 on the transfer belt 4(7) after the rotation of the transfer belt 4(7) is resumed when an image forming process is started, which results in a long time until an image is formed.
In order to make the time shorter in which the sensor 12 detects the home position mark 11 on the transfer belt 4(7) after the rotation of the transfer belt 4(7) is resumed when the image forming process is started, a method is conceived in which the home position mark 11 on the transfer belt 4(7) is detected by the sensor 12 at the time the transfer belt 4(7) is stopped, and the transfer belt 4(7) is stopped such that the home position mark 11 always comes immediately before the sensor 12 based on a detected signal. However, this method has a problem in that the life of the transfer belt is shortened since the transfer belt is rotated more than necessary. Additionally, the method has another problem in that, when the apparatus is stopped since, for example, the cover is opened or a jam occurs while forming an image, it is impossible to set the stop position of the transfer belt to a predetermined position.
Therefore, it is desired to perform speed control of the transfer belt by detecting the current relative position of the transfer belt as quickly as possible after an image forming process is started, irrespective of the stop position of the transfer belt.